Lowery, et al. U.S. Pat. No. 3,492,479 discloses a portable nuclear gauge which utilizes a fast neutron source and a thermal neutron detector for determining the composition of a bulk material, such as a bituminous paving mix, placed in a sample pan. This type of gauge relies upon the neutron moderating characteristics of hydrogen atoms present in the composition for determining, for example, the amount of asphalt in a paving mix or the amount of moisture in a building material. For these determinations it is known that the amount of asphalt and the amount of moisture can be related to the hydrogen content of the material, and the hydrogen content of the material can be determined by subjecting the sample to radiation from a fast neutron source and detecting neutrons which have been slowed or thermalized as a result of interaction with the hydrogen nuclei present in the sample. The number of thermalized neutrons detected over a period of time is counted, and this "count" is utilized in determining the asphalt content of the sample.
In operating the gauge, it is first necessary to establish a standard count for calibration purposes. This is accomplished using a standard sample which contains a layer of material known to produce some standard count, for example, a block of polyethylene. Then calibration curves are produced for the particular material being tested, by using carefully prepared samples of known asphalt content. After the calibration curves have been produced, unknown test samples can be placed in the gauge and counts are taken. By reference to the calibration curve, the corresponding asphalt content for that count can be read.
A more recent model of this gauge has been produced by applicant's assignee embodying the principles of the Lowery patent and sold as the "Model 3241 Asphalt Content Gauge" by Troxler Electronic Laboratories, Inc. This gauge includes a microprocessor to facilitate calibration and computation of the sample asphalt content. Calibration can be made by taking gauge counts on two samples of known asphalt content. The microprocessor then constructs a calibration equation from these data points, and the gauge provides a direct readout of the sample composition (percent asphalt), thus eliminating the necessity of calculations and reference to external calibration tables.
In using the gauge for measuring the asphalt content of hot asphalt samples, it was noted that the temperature of the sample has an effect upon the accuracy of the resulting reading and that variations in temperature from sample to sample can produce error or inaccuracy in the reading. In order to overcome this source of inaccuracy, the aforementioned Model 3241 gauge provided for the operator to manually enter into the instrument the temperature of the sample. Then a correction factor calculated as a function of the temperature was applied by the instrument to the experimental count in order to correct for the temperature effect. The accuracy and reliability of the Model 3241 gauge has been quite satisfactory and this gauge has gained a wide acceptance in the industry. However, applicants have now discovered how to further improve and enhance the accuracy level of the measurements made by the gauge.